I’ve long felt that the indigenous peoples of Taiwan should be included as members of Southeast Asia. There are clear commonalities between their lifestyles and those of people in places like Borneo and the mountains of the Southeast Asian mainland.
The Atayal woman above, for instance, reminds me of pictures that I have seen of tattooed minority women in Burma.
Japanese photographers were among the first people (perhaps the first?) to take pictures of the indigenous peoples of Taiwan. They started to do so not long after they had obtained that island as a colonial possession in the late nineteenth century, when it was known to people in the West as Formosa, and when the “savages” on the island, like this group here, were still culturally very distinct.
That said, it is also clear that there was a lot of cultural mixing that went on, as the attire of this elite Paiwan couple indicates.
With this commoner couple, however, we see the same situation where the woman is dressed in what appears to be a more Sinicized outfit.
This Amis couple is similar.
This headman from Pingdong County, on the other hand, looks defiantly indigenous.
As does this warrior here.
These pictures come from the East Asian Image Collection at Lafayette College. The links above are to pages that contain source information for each picture.
Le Minh Khai's SEAsian History Blog (+ More) 
This is an interesting finding! But could you please explain more about your statement “the indigenous peoples of Taiwan should be included as members of Southeast Asia?” You mean indigenous peoples of Taiwan might have been from SEA? Thank you so much!
First of all, what we today call “Southeast Asia” is a modern invention, so we have to be careful how we use that term.
Basically, I don’t feel satisfied with the way that people have tried to define Southeast Asia (like Anthony Reid in his Southeast Asia in the Age of Commerce). People have tried to make an argument for cultural coherence, but they do so by picking and choosing things from various peoples who (as I see it) “formed” at different times and under different conditions.
I think there was once (let’s say around 500 BC) quite a lot of cultural coherence to an area that stretched from at least the Yangzi River all the way to the islands of what is today Indonesia. What was that culture like? Well a lot of people lived in houses on stilts, or even longhouses. They headhunted. Tattoos were an important part of their culture. . . In other words, this cultural world was a lot like what we can still see traces of in these pictures from Taiwan.
That world changed in many ways, however, as people in this vast region interacted with people and ideas from areas outside this region. As a result, if we were to go back in time say 300 years and visit Ayutthaya, Ha Noi and Melaka, we would find a lot of cultural variation.
However, not everyone changed as dramatically as others. Therefore, if on our trip 300 years ago we also visited the mountains of Taiwan and Sumatra, and the jungles of Borneo, I think we would have found people who were culturally quite similar in many ways, and whose cultures (although having changed plenty through their contacts with others) still contained quite a few elements that were part of the larger cultural world of say 500 BC.
It is of course dangerous to say that there is an “original” culture, or that there are some people who “remain unchanged” etc. However, my sense is that the lifestyle of the people in these pictures is more like the way people lived across the vast area south of the Yangzi and north of Australia before certain people adopted other ideas about how to live.
Many of the people who we call “Southeast Asian” today (the Malays, the Vietnamese, the Burmans, the Thai) are all people who formed new cultures through the adoption and use of cultural practices and beliefs that were not indigenous to the region (Islam, Buddhism, etc.). So to me, they are a different kind of “Southeast Asian” than are the headhunters of Borneo, the Philippines and Taiwan. In many cases, people like the Vietnamese and the Malays defined themselves in opposition to these other peoples.
And as for where the indigenous people in Taiwan came from – that’s a big debate. If you look on the web for the “Austronesian migration debate” or something like that, you’ll easily be able to find more on that. Whatever the case may be, I think it’s safe to say that there was plenty of movement throughout the entire area from the Yangzi to the Indonesian islands starting way back in time.
Thanks for the detailed, understandable and useful reply!